Our Flights Can Break Sound Barrier Without a Sonic Boom

PCMag editors select and review products independently. If you buy through affiliate links, we may earn commissions, which help support our testing.

Boom Supersonic has a change to its flight plan that might also suggest a change to its name is also in order: a “boomless cruise” option in which its planned Overture supersonic airliner would fly a little faster than the speed of sound without a sonic boom reaching the ground.

The Denver startup announced this advance on the morning of the second and final supersonic flight of its XB-1 technology demonstrator out of California’s Mojave Air & Space Port. Two weeks ago, that single-seat, three-engine aircraft became the first independently developed plane to break Mach 1 in level flight.

On Monday, Boom CEO Blake Scholl revealed that XB-1’s pioneering Jan. 28 flight had also successfully tested the boomless-cruise concept. “We broke the sound barrier not once, not twice, but actually three times—each time without making an audible sonic boom,” Scholl said on Boom’s livestream. “It’s about how you fly the airplane.”

As a post on Boom’s site explains, boomless cruise exploits a phenomenon called Mach cutoff, when the sonic boom generated by high-altitude flight at speeds not too far past Mach 1 “refracts upward due to temperature and wind gradients affecting the local speed of sound.”

A NASA study (PDF) of a series of test flights in 2012 with an F-18 fighter jet established that boomless operation was doable with precise measurements and flying: “Airplane speed and altitude will have to be controlled, and real-time atmospheric conditions will have to be monitored, each to great accuracy.”

Monday’s XB-1 flight, the last planned for that jet, saw it exceed Mach 1 three times without a boom being detected by microphones set up on the desert floor below.

At typical speeds between Mach 1.1 and 1.2, Boom says boomless cruise on Overture could knock “up to 90 minutes” off a flight from New York to Los Angeles, typically six or so hours.

To make this doable in routine operation by that four-engine airliner, Boom is banking on the Symphony engines now under development by a consortium of outside manufacturers, which it says will allow Overture to wait to exceed Mach 1 until it’s above 30,000 feet, plus “an advanced autopilot” to calculate the right boomless speed for weather conditions.

Boom is not the first startup to plan on exploiting Mach cutoff for quiet overland supersonic flight: That was part of the plan for Aerion’s AS2 business jet until that firm shut down in May 2021, citing funding difficulties.

But Boom is also banking on a change in the US regulations that today ban civil supersonic flight over land. “We look forward to working with regulators to update these regulations to unlock Boomless supersonic flight over land,” Scholl wrote at the end of a thread on X Monday morning in which he also joked about renaming the company to “Shhh Supersonic.”

Those regulations have no noise threshold, even as both private and government ambitions for quieter supersonic flight have slowly advanced. For example, the X-59 test plane Lockheed Martin is building for NASA’s Quesst project to test low-boom aerodynamics.

“This is a small regulatory change that could drive enormous economic growth,” emailed Eli Dourado, chief economist at the Abundance Institute, a Salt Lake City nonprofit that advocates for emerging technologies. “It would drive industry investment in high-speed technologies and signal the start of the supersonic age.”

In 2016—prior to a 2.5-year stint working on policy at Boom—Dourado wrote a paper (PDF) with Samuel Hammond for George Mason University’s Mercatus Center urging liberalization of those Federal Aviation Administration regulations. The two argued that advances in composite airframe construction would allow lighter supersonic aircraft that would generate less of a boom than the pioneering Concorde airliner, recommending that the FAA rewrite those rules to restrict sound, not speed.

Boom’s plans for overwater flights by Overture—a delta-winged airliner that will accommodate from 64 to 80 business-class passengers—still involve cruising at up to Mach 1.7. Those speeds will send a sonic boom all the way down to the waves but allow Overture to cut flight times between New York and London to roughly 3.5 hours, about half of today’s travel times of six to eight hours between Newark or JFK and Heathrow.

United Airlines signed an order for 15 Overtures in 2021 and American Airlines inked an order for 20 of these jets in 2022, but no other airlines have placed firm orders for Overture since.

And Boom first has to build this plane and test it to the satisfaction of aviation regulators in the US and the EU. On Boom’s stream Monday, Scholl said the company will quickly move to finalize designs of Overture and then its engines.

“We’re literally about a week away from calling design freeze on Overture, we’ll be calling design freeze on our Symphony engines in March,” he said. Then Boom plans to start constructing the first Overture in 18 months and roll it out of its Greensboro, N.C., factory in about three years.

“Our goal is to be ready for passengers by the end of 2029,” Scholl said. But staying anywhere close to that schedule will require a level of precise accomplishment in construction and testing—and in completing the painstaking work of regulatory certification—that seems likely to make boomless supersonic flight look easy.